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  Java Practices -> Conditional compile
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<div class='page-title'>Conditional compile</div>

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<br>The conditional compilation practice is used to optionally remove chunks
of code from the compiled version of a class. It uses the fact that compilers
will ignore any unreachable branches of code.
<p>To implement conditional compilation,
<ul>
<li>
define a&nbsp; <tt>static final boolean</tt> value as a non-private member
of some class</li>

<li>
place code which is to be conditionally compiled in an <tt>if</tt> block
which evaluates the <tt>boolean</tt></li>

<li>
set the value of the <tt>boolean</tt> to <tt>false</tt> to cause the compiler
to ignore the <tt>if</tt> block; otherwise, keep its value as <tt>true</tt></li>
</ul>
This practice is used mostly for implementing debugging statements, logging
statements, and for assertions. With the advent of <tt><a href="TopicAction45c1-2.html">assert</a></tt> and
the <a href="TopicAction2316-2.html">Logging API</a> in JDK 1.4, the utility of conditional compilation is probably
reduced.
<p>Example
<br>
<PRE>

<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> Debug {
  <span class='comment'>//set to false to allow compiler to identify and eliminate
</span>  <span class='comment'>//unreachable code
</span>  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>final</span> <span class='keyword'>boolean</span> ON = <span class='keyword'>true</span>;
} 
</PRE>
<br>
<br>This class uses&nbsp;<tt>Debug</tt> to implement conditional compilation
:
<br>
<PRE>

<span class='keyword'>import</span> javax.servlet.*;
<span class='keyword'>import</span> javax.servlet.http.*;
<span class='keyword'>import</span> java.io.*;

<span class='comment'>/**
* Uses conditional compilation to optionally print debugging statements.
*/</span>
<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> GenericServlet <span class='keyword'>extends</span> HttpServlet {

  <span class='keyword'>public</span> <span class='keyword'>void</span> init(ServletConfig aConfig) <span class='keyword'>throws</span> ServletException {
    <span class='keyword'>super</span>.init(aConfig);

    <span class='comment'>//print out a message, but only if Debug.ON evaluates to true
</span>    <span class='comment'>//if Debug.ON evaluates to false, this entire block of code will be
</span>    <span class='comment'>//passed over by the compiler, and will not be present in output .class file
</span>    <span class='keyword'>if</span> (Debug.ON) {
      System.out.println(<span class='literal'>"**** INIT OF My Controller *****"</span>);
    }
  }

  <span class='keyword'>public</span> <span class='keyword'>void</span> doGet(
    HttpServletRequest aRequest,
    HttpServletResponse aResponse
  ) <span class='keyword'>throws</span> ServletException, IOException {
    <span class='keyword'>if</span> (Debug.ON) {
      System.out.println(<span class='literal'>"Processing doGet"</span>);
    }
    doHttpRequest(aRequest, aResponse);
  }

  <span class='keyword'>public</span> <span class='keyword'>void</span> doPost(
    HttpServletRequest aRequest,
    HttpServletResponse aResponse
  ) <span class='keyword'>throws</span> ServletException, IOException {
    <span class='keyword'>if</span> (Debug.ON) {
      System.out.println(<span class='literal'>"Processing doPost"</span>);
    }
    doHttpRequest(aRequest, aResponse);
  }

  <span class='comment'>// PRIVATE //
</span>  <span class='keyword'>private</span> <span class='keyword'>void</span> doHttpRequest(
    HttpServletRequest aRequest,
    HttpServletResponse aResponse
  ) <span class='keyword'>throws</span> ServletException, IOException {
     <span class='comment'>//empty
</span>  }
} 
</PRE>
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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction45c1-2.html?Id=101'>Assertions in general</a> <br>
 
  
  <a href='TopicAction2316-2.html?Id=143'>Logging messages</a> <br>
 
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